'transform' style for fullscreen video elements

Context: https://bugzilla.mozilla.org/show_bug.cgi?id=1099052
https://fullscreen.spec.whatwg.org/#user-agent-level-style-sheet-defaults

Chrome has "-webkit-transform: none;" for video elements in its UA
stylesheet, though this isn't in the spec. Should we add that to the spec
or try to get the Chrome bug fixed?

Rob
-- 
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